It is well known that for the production of a photographic material, particularly a color photographic material, a multi-layer structure consisting of at least a red-sensitive emulsion layer, at least one more emulsion layer, intermediate layers and the other layers is generally employed. In this case, it is also well known that a spectral sensitization process (i.e., a technique in which some kind of sensitizing dye is added to a silver halide photographic emulsion (which is hereafter referred to as "an emulsion") to extend the light-sensitive wavelength range towards the longer wavelength side) is used in order to give color sensitivities of blue sensitivity, green sensitivity, red sensitivity and the like to each emulsion layer.
The strength of spectral sensitization is influenced by the chemical structure of sensitizing dyes, properties of emulsions (e.g., halogen composition of a silver halide, crystal habit, crystal system, silver ion concentration, hydrogen ion concentration, etc.) and the like. Moreover, the spectral sensitivity is also influenced by photographic additives such as stabilizer, antifoggant, coating assistant, colored coupler and the like which coexist in the emulsion. Among the sensitizing dyes, as a sensitizing dye which affords red sensitivity, for example, the carbocyanine dye containing a quinoline nucleus as described in U.S. Pat. No. 3,556,800, the rhodacyanine dye as described in Japanese Patent Publication No. 4930/68, the melocyanine dye as described in U.S. Pat. No. 3,416,927, the dicarbocyanine dye as described in U.S. Pat. Nos. 2,503,776 and 3,635,721 and Japanese Patent Publication No. 550/71, or "The Cyanine Dyes and Related Compounds" by Hamer (1964), p. 207, and the like have been known.
However, it is also well known that when these sensitizing dyes are added to the emulsion, a supplementary effect which is photographically detrimental is caused. Detrimental supplementary effects include the occurrence of fog which results from the addition of a sensitizing dye (which is hereinafter referred to as "dye fog") and diffuse sensitization [this means that the sensitizing dye does not stay only in the emulsion layer in which the dye is first added, diffusing to another emulsion layer layer over time, and thus an unpreferable sensitization is caused by the diffusing dye in the diffused layer (which is hereinafter referred to as "diffuse sensitization")]. They are important obstacles in the production of multilayer color photographic materials having high spectral sensitivity.
In dye fog, when the used photographic material is a photographic paper, even a small increase in fog is not desirable and remarkably decreases the commercial value. In order to improve the dye fog, when a fog inhibitor or stabilizer which are generally used is used together with the sensitizing dye, the fog can be inhibited. However, this causes a lowering of the spectral sensitivity in some cases or an increased decline of the spectral sensitivity over time. Accordingly, this kind of fog inhibitor is required a strict selectivity against the spectral sensitizing dye used.
In diffuse sensitization, the sensitizing dye added to a red-sensitive emulsion layer diffuses in a green-sensitive silver halide photographic emulsion layer (which is hereafter referred to as "a green-sensitive emulsion layer") and/or a blue-sensitive silver halide photographic emulsion layer (which is hereafter referred to as "a blue-sensitive emulsion layer"), and the diffusing dye causes a panchromatic sensitization, resulting in color mixing (turbidity) of the color image, a lowering of the green sensitivity of the green-sensitive emulsion layer and a lowering of the blue sensitivity of the blue-sensitive emulsion layer. Accordingly, the diffuse sensitization causes serious problems with respect to photographic efficiency.
Accordingly, to develop a spectral sensitizing dye which hardly affords diffuse sensitization has been an important object of persons skilled in the art.
On the other hand, the desorption or decomposition of the sensitizing dye or a change of the absorpting condition in the emulsion solution before coating causes a change in spectral sensitivity or an increase in fog over time before coating (hereinafter called to as "lapse of dissolution property"). This fact is an important obstacle for the stable production of a photographic material having high sensitivity.
Melocyanine dyes, particularly tetramethine melocyanine dyes, are generally superior with respect to their color sensitizing property (sensitivity), spectral sensitivity distribution, printer suitability, temperature and humidity dependence at exposure, solubility of dye and the like, compared with the carbocyanine dye containing a quinoline nucleus, the rhodacyanine dye and the dicarbocyanine dye which are similarly used for red color sensitization.
Furthermore, the tetramethylene melocyanine dye is desirable because it hardly causes dye stains after development. On the other hand, however, it has disadvantages in that the diffuse sensitizing property is large, the sensitivity in the lapse of dissolution is widely lowered and the dye fog is large.
Accordingly, it is difficult in fact to use tetramethine melocyanine dye by itself due to these difficulties. However, if these difficulties can be removed by any device, the tetramethine melocyanine dye can become a quite useful red sensitizing dye.
Examples using melocyanine dyes are described in U.S. Pat. Nos. 3,416,927, 4,002,480 and the like. Using a tetramethine melocyanine dye together with a s-triazine compound is described in U.S. Pat. No. 3,416,927. However, although the sensitivity is higher than when using the dye alone, the dye fog and the diffuse sensitization are not sufficiently improved. Using a melocyanine dye together with a compound containing a pyrimidine nucleus is described in U.S. Pat. No. 4,002,480. However, although the dye fog and the like are improved, the lapse of dissolution property is not improved sufficiently and thus the diffuse sensitizing property must be further improved.